The invention relates to annealing and more particularly to annealing of single crystals to yield single crystals with low stress birefringence such as for use as optical lenses.
The increase in the processing speed, functionality, and integration in integrated circuits (ICs) has been achieved through continuous reduction in the feature sizes of the ICs. A portion of the manufacturing of the ICs affecting attainable feature sizes is photolithography. During photolithography, a pattern of the IC is transferred from a mask to a wafer, e.g., a semiconducting wafer. Imaging characteristics of modern projection optical photolithography equipment are dominated by diffraction effects. The resolution (i.e. the smallest feature size that can be printed on the wafer) is k1 xcex/NA, where xcex is the wavelength of the light source, k1 is a constant approximately equal to 0.5, and NA is the numerical aperture of the projection optics. The depth of focus of the projection printer over which the image quality is not degraded is limited and is equal to k2 xcex/(NA)2, where k2 is a constant that depends on k1. Thus, to decrease the feature size either the wavelength of exposure must be reduced or the NA of the optics must be increased.
Increasing the optics NA to reduce feature size results in a substantial reduction in the depth of focus (xcx9c(NA)xe2x88x922), which is undesirable, particularly because the depth of focus must be larger than any variations in the flatness of the photoresist surface. Therefore, the semiconductor industry is pursuing the use of short wavelength exposure sources for achieving smaller and smaller feature sizes. KrF, ArF, and F2 excimer lasers are presently available as light sources for, respectively, 248, 193, and 157 nm photolithography. The synthetic fused silica, however, that has been the optical material of choice for higher wavelength exposure sources, exhibits significant loss of transmittance at wavelengths below 200 nm.
Single crystals of Calcium Fluoride (CaF2) exhibit the desirable optical properties for sub 200-nm-photolithography. Furthermore, for historical reasons the production knowledgebase for CaF2 is relatively extensive. Other single crystals of fluoride such as BaF2 and LiF are also possible material candidates, but are significantly behind CaF2 in production technology, and may be less desirable, e.g., due to toxicity and corrosiveness (BaF2) and/or expense (LiF). Therefore, single crystal CaF2 are desirable and suitable optical material for 193 and 157 nm optical steppers. Presently, CaF2 crystals as large as 30 cm in diameter and 10 cm in height are used in photolithography equipment.
Single crystals of CaF2 are grown by directional solidification from the melt phase. In this process layers of the melt are continuously solidified, by changing the temperature of the crystal, to form a single crystal boule. The crystal boule is subsequently cooled to room temperature. The transfer of heat from and through the crystal sets up temperature gradients (i.e. temperature non-uniformities) and associated thermal stresses in the single crystal. CaF2 is a relatively weak material, especially at elevated temperatures, and therefore experiences plastic deformation under thermal stresses during the crystal growth process. The accumulation of plastic strain during the crystal growth process results in generation of residual stresses in the crystal at room temperature. Residual stresses, in turn, cause stress birefringence through spatial variations in the material""s index of refraction, and an associated degradation of optical characteristics of components made from this material.
Annealing is used to reduce residual stresses in crystals that have experienced plastic deformation during the crystals"" growth process. To anneal a crystal, the crystal is maintained at an elevated temperature close to its melting point temperature for a period of time. This constant temperature is intended to allow existing residual stresses to relax. The crystal is cooled to room temperature. During cooling, temperature gradients associated with the cooling of the crystal generate thermal stresses in the crystal that may cause the crystal to undergo plastic deformation.
Due to the nature of the material, temperature variations to which a single crystal is exposed to during growth and annealing result in large thermal stresses leading to plastic deformation of the crystal and, hence, large residual birefringence.
In general, in an aspect, the invention provides a system for heating optical members. The system includes a thermally-conductive inner housing defining an interior volume for receiving an optical member to be heated, a thermally-insulative outer housing at least partially containing the thermally-conductive inner housing, and a heating structure disposed outside the inner housing and configured to provide heat through the thermally-conductive inner housing and into the interior volume defined by the inner housing.
Implementations of the invention may include one or more of the following features. The inner housing is configured such that an inner surface defining the interior volume has a substantially uniform temperature in response to the inner housing receiving the heat provided by the heating structure. The inner housing is configured to define the interior volume to be axi-symmetric.
Further implementations of the invention may include one or more of the following features. The system further comprises a controller coupled to the heating structure and configured to control the heating structure such that the member disposed in the interior volume is heated substantially without being plastically deformed. The controller is configured to control the heating structure such that a resolved shear stress of a CaF2 optical member disposed in the interior volume does not exceed about 0.5 e(990/T) MPa where T is average temperature of the member in Kelvin.
Further implementations of the invention may include one or more of the following features. A portion of the outer housing in contact with and supporting the inner housing has a thermal conductivity different than at least one other portion of the outer housing. An inner boundary of the outer housing is disposed in contact with substantially an entire outer boundary of the inner housing. The inner housing and at least a portion of the outer housing are an integral structure, with the inner housing and the at least a portion of the outer housing being layers of the integral structure with different thermal conductivity.
Further implementations of the invention may include one or more of the following features. The inner housing comprises at least one of high-thermal-conductivity graphite and high-thermal-conductivity carbon. The interior volume is cylindrical and directions of highest thermal conductivity of the inner housing are parallel with inner surfaces of the inner housing. The interior volume is cylindrical and directions of lowest thermal conductivity of the inner housing are perpendicular with inner surfaces of the inner housing. Directions of lowest thermal conductivity of the outer housing are perpendicular with outer surfaces of the inner housing.
Further implementations of the invention may include one or more of the following features. The inner housing has substantially orthotropic thermal conductivity. The outer housing comprises at least one of low-thermal-conductivity graphite, low-thermal-conductivity carbon, low-thermal-conductivity porous graphite, low-thermal-conductivity porous carbon, low-thermal-conductivity fibrous graphite, low-thermal-conductivity fibrous carbon. The outer housing has substantially orthotropic thermal conductivity. The system further comprises another thermally-conductive housing, the another thermally-conductive housing substantially contains the thermally-insulative outer housing. The another thermally-conductive housing is displaced from the outer housing.
Further implementations of the invention may include one or more of the following features. The inner housing defines a plurality of interior volumes each for receiving an optical member to be heated. The inner housing has a substantially isotropic thermal conductivity. The outer housing has a substantially isotropic thermal conductivity. At least a portion of the heating structure is disposed outside the outer housing.
In general, in another aspect, the invention provides a method of heating an optical member. The method includes providing the optical member, directing heat from a heat source toward the optical member, and distributing the heat about the optical member through a high-thermal-conductivity apparatus disposed between the heat source and the optical member such that a surface of the apparatus defining a volume for receiving the optical member will have a substantially uniform temperature.
Implementations of the invention may include one or more of the following features. The heat is distributed such that temperatures of the surface of the apparatus defining the volume vary by no more than about 0.5 K where K is temperature in Kelvin. The method further comprises measuring at least one indication of temperature of the apparatus defining the volume. The at least one indication includes a plurality of indicia of temperature of the apparatus, the indicia being related to at least one of an outer surface, an inner surface, and an interior of the apparatus. The method further comprises adjusting how much heat is directed toward the optical member in accordance with the at least one indication. The adjusting is in accordance with a model of temperature variations within the optical member. How much heat is directed toward the optical member is adjusted to guard against stress within the optical member exceeding a critical resolved shear stress of the optical member during at least one of annealing of the optical member and cool down of the optical member.
Further implementations of the invention may include one or more of the following features. The method further comprises inhibiting heat from transferring away from the optical member from the high-thermal-conductivity apparatus. A plurality of optical members is provided, wherein heat is directed from a heat source toward each of the optical members, and wherein the heat is distributed about each of the optical members through the high-thermal-conductivity apparatus disposed between the heat source and the optical members such that surfaces of the apparatus defining volumes for receiving the optical members will each have a substantially uniform temperature.
In general, in another aspect, the invention provides a system for annealing at least one single crystal blank for use as at least one optical lens. The system includes a heating structure for supplying heat, heating means for heating the at least one single crystal blank, using the heat from the heating structure, to an annealing temperature of the blank and for cooling the at least one single crystal blank from the annealing temperature to an ambient temperature substantially without plastic deformations developing in the at least one blank, the heating means including at least a high-thermal-conductivity housing for containing the at least one single crystal blank.
Implementations of the invention may include one or more of the following features. The heating means further includes an insulator structure at least partially containing the high-thermal-conductivity housing. The heating means further includes a controller coupled to the heating structure for regulating heat provided by the heating structure to permit annealing of the at least one blank while inhibiting temperature gradients inside the at least one blank from producing plastic deformations. The heating means further comprises temperature sensors coupled to the controller configured to provide indicia of temperatures of the high-thermal-conductivity housing to the controller and wherein the controller regulates the heat provided by the heating structure in response to the indicia provided by the temperature sensors. The controller inhibits temperature gradients inside each of the at least one blank from producing stresses in excess of about 0.5 e(990/T) MPa where T is average temperature of each blank in Kelvin.
In general, in another aspect, the invention provides an optical member including a single crystal material substantially free of residual stress and having an optical birefringence of less than about 1 nm/cm.
Implementations of the invention may include one or more of the following features. The single crystal material forms an optical lens blank. The single crystal material is a fluoride. The single crystal material is CaF2.
Various aspects of the invention may provide one or more of the following advantages. A substantially isothermal environment may be provided for members, such as optical blanks or lenses, to be annealed (during annealing and cool down), or otherwise heat treated. Temperature nonuniformities along walls of a chamber containing a member to be heated can be reduced relative to prior systems. Heat loss through a support structure for supporting a chamber to contain a member to be heated can be reduced relative to prior systems. Time-dependent variations of temperature on an interior portion of a container of a member to be heated can be dampened relative to corresponding time-dependent variations on an exterior portion of the container. Radial temperature variations within an axi-symmetric crystal can be kept below a level that would induce stresses exceeding a critical resolved shear stress of the crystal during an annealing and/or cool down period. Annealed items, e.g., optical members, can be produced with low birefringence, e.g., less than about 1 nm/cm.
These and other advantages of the invention, along with the invention itself, will be more fully understood after a review of the following figures, detailed description, and claims.